2 Answers
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Stack Overflow was made to be fast because a quick and responsive browsing experience really improves the usability of the site.

In general, faster is better. That's why it is optimized for speed.

As for how, cache, cache, cache. Just about every page you will normally hit is a cached version. This really reduces load on the servers. They also do things like using one image for most of the small icons. This reduces the number of resources required to download.

And, they have a special cookie-free domain (sstatic.net) for hosting their static content. That really speeds up the static content.